Method of making chromium-nickel austenitic stainless steel



Patented Apr. 24, 1945 METHOD OF MAKING CHROMIUM-NICKEL AUSTENITIC STAINLESS STEEL Stephen F. Urban, Chicago, Ill.

No Drawing. Application May 9, 1941,

Serial No. 392,779

2 Claims.

This invention relates to the making of chromium-nickel austenitic stainless steel, this application being a continuation-in-part of a copending application filed'March 7, 1940, and bearin Serial No. 322,843.

The accepted current commercial method of making such steel is by refinement in an electric furnace, usually an arc furnace, of a molten ferrous charge by means of iron-oxides, the carbon being reduced in this manner and the charge being subsequently deoxidized and provided with the necessary alloys. Its deoxidation is effected by ferrosilicon and ferromanganese which add carbon, and the necessary nickel and chromium additions also add carbon to some extent, it following that the ultimate steel has a higher carbon content than that of the charge immediately after the refinement period and before any additions are made.

As is well known, the above method cannot produce a chromium-nickel austenitic stainless steel containing less than .05% carbon and even this low a carbon maximum cannot ordinarily be obtained in a commercially practical manner, the refinement period being excessively long and an excessive amount of slag being produced, these factors increasing the furnace operation and maintenance costs to a degree where the cost of the steel prohibits its use.

Keeping the above in mind, a chromium-nickel austenitic stainless steel containing as much as .05% carbon and which is heated to within its carbide precipitation temperature range and exposed to corrosive conditions, corrodes very rapidly at its grain boundaries, this effect being called intergranular corrosion and being attributed to carbon precipitating at the grain boundaries of the steel. Since the trouble is caused by carbon in the steel, it follows that if the carbon could be reduced to a sufficiently low amount, a steel that is insensitive to intergranular corrosion would result, it having been ascertained that a chromium-nickel austenitic stainless steel with .030% carbon maximum is insensitive to intergranular corrosion. This factor represents a fixed maximum above which the carbon content of the steel must not go, and if such a steel is to be produced commercially, the aim must be to obtain a .025% carbon content and in no instance to allow the carbon to go above .030%.

that the idea is impractical.

Prior to the present invention, those skilled in the art have been unable to provide a commercially practical method of making a steel of the character under discussion with such a low carbon content. Therefore, it has been necessary to solve the problem of intergranular corrosion by another method, namely, by the use of carbide stabilizing alloys, such as columbium, titanium, etc. which fix the carbon so as to prevent its solution and reprecipitation. These alloys are expensive and, what is even more important, they introduce difflculties during the hot working of the steel, it being well known, for instance, that these alloys cause trouble during seamless tube piercing. However, prior to the present invention, this alloy stabilized type of steel has been the only one that could be commercially provided for use under conditions where intergranular corrosion is encountered, this being because of the inability of the prior art methods to provide a commercially practical method of making chromium-nickel austenitic stainless steel with a positive carbon maximum of .030%.

Since it is oxygen that reduces the carbon of the charge during the refinement period, it is obvious that oxygen-bearing materials other than iron-oxides might be used to remove the carbon from the ferrous charge in the furnace, providing their use proved advantageous. One material that has been suggested by the prior art is nickeloxlde, the thought being advanced that this material might be advantageously substituted for the currently used iron-oxides plus nickel metal because its use would not only eliminate carbon in the same manner but would also result in nickel being simultaneously added to the bath. However, when the very high cost of nickel-oxide is balanced against the convenience attendant its use as a simple substitute for 'iron-oxides-plus nickel metal, it immediately becomes apparent In other words, when used to produce a chromimum-nickel austenitic stainless steel with the analysis comparable to that produced by the use of iron-oxides,

the price that steel makers must pay for nickeloxides is so high as to make its use prohibitive. In appreciation of this, the suggestion has also been advanced that impure nickel-oxide be used,

- the 030% maximum that must 2 I austenitic stainless steel cannot be produced y this practice-because there is no practicalway .of removing the copper.

Because of the above, no commercial advantare has been ever gained from the prior art suggestions respecting the use of nickel-oxide for the purpose of reducing the carbon during the refinement period involved in the production of chromium-nickel austenitic stainless steels.

Contrary to all ofthis, the present inventor has discovered that when nickel-oxide is used in a ferrous carbon-containing charge maintained molten in an electric induction furnace; that the nickel-oxide provides the new and unexpected result of dropping the carbon content of the charge,,even when this content is very high, to a sufficiently low carbon content to permit subsequent deoxidation with ferrosilicon and ferromanganese, and the use of all the various alloy additions'necessary to produce any of the series of chromium-nickel austenitic stainless steels with a carbon content of around .025% and so that in all events the carbon can be kept safely below be observed if intergranular corrosion is to be avoided to a satisfactory degree. ed that nickel-oxide can be used to produce a ferrous bath having a much lower carbon content than can possibly be produced by iron-0xides. It is to be observed that this inventor has .found that nickel-oxide is not merely a substitute for iron-oxides and nickel, but that its use involves the new and unexpected result that carbon contents can be obtained in a commercially practical manner that are much lower than has heretofore been considered a possibility in electric induction furnace practice.

In addition to this, it has been discovered that the above described use of nickel-oxide shortens the refinement period so much and produces such a small amount of slag, as compared to the use of iron-oxides, that electric furnace operating and maintenance costs are reduced to such an extent that a steel can be produced that is as insensitive to intergranular corrosion as is the alloy stabilized type, more economically than can be done by the prior art method of producing the alloy stabilized type in spite of the high price that must be paid by the steel makers for nickeloxide. Thus, although it is commercially impos- Heretofore no one has suspectsible to use nickel-oxide as a substitute for ironpossible and very practical to use nickel-oxide not as a substitute for iron-oxides plus nickel metal, but as the only medium by means of which the carbon in the usual ferrous charge maintained molten in an electric induction furnace, can be reduced to a sufllciently low factor to permit the subsequent necessary additions to result in a chromium-nickel austenitic stainless steel having a carbon content positively maintained at a maximum of .030% and which normally may be expected to range around .025% or less. Such a steel is produced in accordance with this invention more economically than it is possible to produce by the prior art methods a chromium-nickel austenitic stainless steel having a higher carbon content and rendered equally insensitive to intergranular corrosion resistance by the use of alloys, such ascolumbium, titanium, etc. Furthermore, the steel resulting from the practice of the present invention has the great advantage that it can be. hot worked -'in any manner without the diiiiculties always attendant :hne use of the commonly used carbide stabilizing Due to the shape of an electric furnace it is diilicult to obtain large carbon removals by the use oi iron-oxides because these float on the bath surface which is of restricted area, where they function mainly to produce large slag volumes during efforts to further reduce the carbon below the usually sought for reductions by greatly prolonging the refining period. It is believed that the new and unexpected result attendant the use of nickel-oxide is due to this material being extremely soluble in -the molten ferrous bath, whereby it can be dissolved very rapidly by the latter so that the reaction proceeds simultaneously throughout the entire bath without producing large slag volumes. This characteristic of nickel-oxide was heretofore unknown.

As illustrative of the diillculties attendant the use of iron-oxides in even the laboratory production of a ferrous bath of extremely low carbon content approaching that required to form a basis for making chromium-nickel austenitic stainless steel containing not more than .030 carbon, the results of a test heat made in a 30 lb. electric induction furnace are advanced in Table 1 which follows this paragraph. This heat started out as a steel bath which melted at 12% carbon and .15% silicon, 55 lbs. of iron ore per ton of the steel being added to this bath, it then being held for 10 minutes, sampled and analyzed, 27 lbs. per ton of iron ore being next added, the bath then being held for another 10 minutes and again sampled and analyzed, and so on for two more times with the following results:

Table 1 Analysis of bath at time shown Ore addition Time Per cent Per cent O Si None 0.12 0. 15 55 lbs. per ton 0. l1 0. 064 27 lbs. per ton. d 0. 034 0. 028 Do 0.022 0. 026 Do ..do 0.028 0 025 Totals, 136 lbs. per ton.. 40 min Remembering that the furnace used was of the induction type so that no carbon could be added, it becomes apparent that even in this laboratory production of a relatively low carbon bath, the refinement period covered a considerable length of time, much longer in fact than could ever be commercially used. In addition, large quantities of slag were produced during this refinement period which would rapidly ruin the lining of any furnace and which would make furnace maintenance costs impossibly high. Furthermore, it is to be noted that the carbon was dropped only to a .028% factor, and this is insufliciently low to permit the production of the desired steel, the silicon, manganese, nickel and chromium which are necessary to produce a chromium-nickel austenitic stainless steel, all introducing a certain amount of carbon which is sufllcient to raise the carbon content of the ultimate steel above .030%. It will be remembered that this is the maximum carbon content permitted. In other words. a .030% factor is quite definitely a maximum above which the carbon must not so if the steel is to be adequately insensitive to intersranular corrosion.

Using the same 30 lb. electric induction furnace, data were collected to illustrate the new and unexpected results attendant theus'e of nickeloxide, this data being advanced by the following tenance costs completely offset the greater cost of the nickel-oxide as compared to the use of ironoxides and nickel alloys, the carbon reduction being greater in any event than can possibly be table: 5' attained by the use of iron-oxides. In other Table 2 Time in mhr v F1118] analysis Pa to Heat 0 flaw: :11} i i i it p Per r a it n or er M I cent 0 cent qr cent Ni A High Carbon scrap 020 M0- No I N B 50% low carbon and 607 high carbon scrap 10 0.00 0.00 :No N Low carbon scrap plus Nias i0 10 0.014 0.022 17.13 9.09 Same as above excegt 8% of Ni as Ni0 10 0.008 0.020 18.11 8.08 E wzhiowNciabcn an 50% high carbon scrap with 8% 10 0.000 0. 019 v 18. 49 8.02

a! F High carbon scrap'with 8% Ni as NiO ":2 l0 0. 010 0.026 18. 21 7. 00

I High carbon, plain carbon steel scrap containing 0.35% (land 0.22% Si. Y Low carbon, plain carbon steel scrap containing 0.03% C and 0.01% Si.

In 'explanationof the table, heats A and B are advanced to show that in the absence of the use of nickel-oxide there is relatively no carbon loss due to maintaining the charge molten in the particular furnace used. Heats C and D show the incredibly low carbon maximums obtained by the use of nickel-oxide in the case of a low carbon scrap charge heat E being even more convincing in that the charge was half high carbon scrap while heat F is truly noteworthy in that with a charge consisting entirely of high carbon steel scrap, the remarkably low carbon maximum of .016% was obtained. In all instances where nickel-oxide was used, a steel of the 18-8 type was ultimately produced having a carbon maximum well below that maximum found necessary to render the steel insensitive t the trouble discussed.

Although not previously emphasized, the present invention contemplates only the use of an electric induction or other furnace operated under conditions preventing the addition of carbon from the heat source, in conjunction with the use of the nickel-oxide, it being impossible to produce a steel having the 025% carbon content at which this invention aims, or with a .030% carbon maximum, when an electric arc furnace is used so as to add carbon in addition to that initially in the charge.

With the foregoing in mind, the present invention may be summarized by saying that its object is to produce a chromium-nickel austenitic stainless steel preferably containing not more than .025% carbon and with a positively fixed carbon maximum of .030%, this steel being insensitive to intergranular corrosion to the same extent as is the alloy stabilized type, but being free from the latters disadvantages, the production of this steel to be in a commercially practical manner and so as to produce it at less cost than the alloy stabilized type could heretofore be produced. This objectis attained by charging an electric induction furnace or other furnace capable of a similar function with a molten ferrous charge containing the usual amounts of carbon and dissolving nickel-oxide in this charge so as to very rapidly reduce its carbon content to 'an amount that is sufllciently low to permit the addition of the carbon containing alloys necessary to produce the steel desired without raising the carbon content of the ultimate steel above the named values. Obviously, no iron-oxides should be used, at least in material amounts. The resulting carbon drop to the desired maximum is so rapid and is attendant by so small an amount of slag that the savings in furnace operation and mainwords, the nickel-oxide is not used merely as a substitute for the use of iron-oxides and nickel, but it performs a function that is entirely new and unexpected in the art. After decarburizing, the charge is deoxidized with ferrosilicon, for instance, and the necessary chromium addition made. As a, matter of convenience, all th nickel may be added as nickel-oxide, it being understood that mail events suilicient nickel-oxide must be used to produce the inventors new and. unexpected result. In the foregoing manner it is possible to attain the object desired. Due to the savings the inventor has taught are involved by the use of nickeioxide, this oxygen-bearing material is now being used in the commercial production of steel for the first time. The specific illustrations of the invention have been of the 18-8 type of steel, but the present invention is applicable to the production of all of the series of chromium-nickel austenitic stainless steels. While for obvious reasons, steel produced in accordance with the present invention need not and should not contain any alloy used for the purpose of carbide stabilization, it is to be understood that certain of the carbide stabilizing elements may be used because they perform other functions, molybdenum, for instance, being useful for such other purposes.

Although pecific examples of the invention have been discussed in connection with the use of a 30 lb. electric induction furnace, the invention is being used commercially to provide the trade for the first time with a chromium-nickel austenitic stainless steel having a carbon content not exceeding .030% and which is completely insensitive to intergranular corrosion. This commercial production involves the use of an 8000 lb. electric induction furnace, this being the largest of this type of furnace used in this country.

It is to be understood that the charge used in carrying out this invention must not contain material amounts of oxidizable elements such as chromium, silicon, etc., which can function to reduce the nickel oxide and prevent the desired rapid and thorough carbon removal, while producing the undesired large slag volumes. Of course, small amounts of such oxidizable material will not render the new method inoperative, plain carbon steel scrap, for instance, containing residual silicon in small amounts resulting from its stainless steel scrap, being avoided for reasons that are now obvious.

I claim:

l. A commercially practical method 0! making an austenitic stainless steel or the chromiumnickel type with a sumciently low carbon content as be inherently insensitive to intergranular cor- -osion, said method incl iding dissolving nickeloxide in a molten carbon-containing ferrous charge in an electric induction furnace under conditionsvreducing the carbon of said charge to a content sutllciently low to permit the subsequent addition of the ferroalloys necessary to produce said type from said charge without raising its carbon content above .030%, and subsequently deoxidizing said charge and adjusting its composition to produce said type of steel of the analysis desired and not more than a .030% carbon content.-

2. A commercially practical method or making an austeni-tic stainless steel or the chromiumnickel type with a sumciently low carbon content to be inherently insensitive to interzranular corrosion, said method including dissolving nickel-oxide in a molten plain carbon steel charg in an electric induction furnace under conditions reducing the carbon or said chme to a content sumciently low to permit the subsequent addition Of the term-alloys necessary to produce said type from said charge'without raising its carbon content above .030%, and subsequently deoxidizing said charge and adjusting its composition to produce said type of steel 0! the analysis desired and not more than a .030%

carbon content.

STEPHEN F. URBAN. 

